Splicing optical fibers using laser light can be traced back to the pioneering work two decades ago, see K. Kinoshita and M. Kobayashi, “End preparation and fusion of an optical fiber array with a CO2 laser”, Appl. Opt., Vol. 18, No. 19, pp. 3256–3260, 1975, and H. Fujita, Y. Suzaki and A. Tachibana, “Optical Fiber Wave Splitting Coupler”, Appl. Opt., Vol. 15, No. 9, pp. 2031–2032, 1976. The concept using a CO2 laser as a heat source for splicing optical fibers was disclosed in French patent FR-2323646, May 21, 1977, inventors Hiroyuki Fujita et al. Apparatus designed for splicing trunk fibers and multi-fibers using a CO2 laser was invented in 1981 and 1982, respectively, see U.S. Pat. No. 4,288,143, Sep. 8, 1981, for Pietro Di Vita et al., and U.S. Pat. No. 4,350,867, Sep. 21, 1982, for Kyoichi Kinoshita et al. An automated laser splicing system was introduced in 1991, see U.S. Pat. No. 5,016,971, May 21, 1991, for Hui-Pin Hsu et al. A number of extended applications related to techniques of laser splicing were also proposed, e.g. restoring carbon coating films on optical fibers using reactant gas and laser to improve tensile strength and fatigue, see U.S. Pat. No. 4,727,237, Feb. 23, 1988, for Christopher A. Schantz, achieving high-strength splices with the assistance of sulphuric acid stripping and laser, see U.S. Pat. No. 4,971,418, Nov. 20, 1990, for Carl S. Dorsey et al., and repairing micro-cracks in and improving the mechanical strength of aged fibers with laser light, see U.S. Pat. No. 5,649,040, Jul. 15, 1997, for Göran Ljungqvist et al.
Fusion splicing using laser light has many advantages over conventional methods, such as methods of fusion splicing using the heat in an electric arc, mechanical splicing, splicing using a hydrogen/oxygen flame, etc. This is because the laser can deliver an intense light beam of high energy and having a high uniformity and repeatability in a very localized area and therefore it can be used for processes requiring a high accuracy, e.g. for high precision cutting of optical fibers, see the published European Patent Application No. 0987570, inventor Henricus Jozef Vergeest. Due to the absence of electrodes or filaments such as used in fusion processes using an electric arc, the laser is considered to be a “clean heat source” which does not contaminate splicing joints and it is, therefore, believed to be the most suitable heat source for high-strength splicing.
Though significant progress in splicing technology using laser light was achieved in the past two decades, industrial applications of laser splicing of optical fibers are still limited. No commercial laser splicers are, at present, available in the market. This might be due to primarily technical reasons, e.g. high demands on the quality of laser beam, on the beam alignment and control systems, on the protection of operators to the laser radiation, etc., and a poor understanding of the rather complicated nature of splicing processes using laser light. Thus, there is a need in the art to establish general concepts of the way in which a fusion splicer should be constructed that uses laser light and allows that controllable fusion processes can be automatically performed in order to handle different fusion processes for all types of optical fibers. The design of a splicer using laser light should also fulfill the requirements for large-scale manufacture, e.g. the splicer should be small, compact, robust, totally safe for operators and it should be easily served and maintained.
The understanding of the fusion process of splicing using laser light is very important for constructing a splicer using laser light. In a conventional splicer, e.g. a fusion splicer using an electric arc, the high temperature needed for splicing, over 1800° C., is mainly obtained by an electric arc that creates a plasma from residue gases, e.g. air, surrounding the optical fibers, whereas the fusion processes using laser light can be mainly attributed to strong absorption of the energy of the laser light directly in the fibers to be spliced. The experimental evidence for supporting the process of optical absorption in splicing using laser light is the weak dependence of fusion temperature on changes of environment, e.g. altitude, humidity etc., and the strong dependence on the operating wavelength of the laser sources used.
Light emitted by CO2 lasers is known to be strongly absorbed by many complex substances, e.g. paper, wood, ceramics, plastic, glass, liquids, granite etc. To date, the CO2 laser is the only laser practically used for splicing optical fibers. In conventional systems, CO2 lasers having an operating wavelength of 10.6 μm are used. FIG. 1 shows infrared absorption spectra obtained from germania-doped silica glass, GeO2—SiO2, phosphosilicate glass, P2O5—SiO2, borosilicate glass, B2O3—SiO2, and fused silica, see H. Osanai, T. Shioda, T. Moriyama, S. Araki, M. Horiguchi, T. Izawa, and H. Takata, “Effect of Dopants on Transmission Loss of Low-OH-Content Optical Fibers”, Electron. Lett., Vol. 12, No. 21, pp. 549–550, 1976. It can be observed that at the wavelength of 10.6 μm, a relative weak absorption of about 15% is obtained for silica glass. From close inspection it is found that, at this wavelength of 10.6 μm, the absorption strongly depends on the different dopants in the fibers and the absorption varies in the range of 10–30%. This means that fusion processes and the physical characteristics and parameters thereof are strongly dependent on the actual type of fibers to be spliced, e.g. the homogeneity of and the quantity of different dopants in the fibers. Furthermore, since the wavelength of 10.6 μm is located in a region where the absorption for the mentioned glasses has a very steep change with the wavelength, small deviations of wavelength, that for example are obtained in the manufacture of CO2 lasers—typically there exists a deviation of about ±0.3 μm—can result in up to 20% drift in absorption. This implies that the optimized fusion parameters may work perfectly for one splicer, and that they might be completely unapplicable in another splicer. Therefore, in order to achieve large-scale manufacture of identical splicers, high demands on the manufacture of identical lasers have to be set, e.g. requiring a high accuracy as to the operating wavelength and a high stability as to optical power issued, which might not be realistic.
Various optical arrangements for splicers using laser light have been proposed in the art, see e.g. U.S. Pat. No. 5,161,207, Nov. 3, 1992, for Joseph L. Pikulski, and U.S. Pat. No. 5,339,380, Aug. 16, 1994, for Joseph A. Wysocki et al. In these patents two types of beam expanders and beam forming apparatus are disclosed. The first patent mentioned uses movable mirrors to deflect a collimated beam to form a diverging conical beam, which is then reflected by a paraboloid mirror to form a convergent conical beam that is in turn focused towards the optical fiber. The second patent uses a beam expander to expand the beam width of a collimated beam, which is then reflected by a paraboloid mirror that focuses the beam towards the fiber joint. For both these patents the splice position of the fibers is located inside the unfocused part of the beam. For the first patent, an indirect alignment of the beam emitted by the CO2 laser is performed by visual observation of the beam of a helium-neon laser of low power. The beam of the helium-neon laser may then be switched alternately into the same beam path as that of the of CO2 laser via a removable mirror. When this mirror is in place it also blocks the light from the CO2 laser, if any. For the second patent the alignment of the CO2 laser beam is controlled by sensing the amount of light that is emitted by the CO2 laser and is scattered to the side from the splice position.